1. Field of the Invention
This invention relates to mixers in general and more particularly to a field effect transistor mixer that has high IP3 and provides wide bandwidth in a small package size.
2. Description of Related Art
A mixer circuit converts a radio frequency (RF) signal to an intermediate frequency (IF) signal which is the difference of the RF and a local oscillator (LO) signal. The IF frequency is obtained by multiplying the RF signal with the local oscillator (LO) signal. The difference or IF frequency is a result of the non-linearity of the mixer. Along with the IF frequency, the mixer typically generates inter-modulation products due to the non-linearity response.
The communication systems of today operate in a multi-carrier environment. When more than one carrier arrives at the receiver, they mix and generate inter-modulation products. One inter-modulation product that causes problems is the third order inter-modulation product. The third order product is close to the carrier frequency and can be difficult to filter out. A measure this third order product is called IP3. Good design techniques can be used to build a high IP3 mixer circuit. Below 2 GHz, such mixers are produced using a ferrite balun transformer. Unfortunately, the ferrite balun transformer is difficult to repeatedly produce. Variations in tightness of the windings, magnet wire twist rate and permeability/permissivity of the ferrite material cause a lowering of the IP3 value. The ferrite transformers are large and require excessive circuit board space. In addition the ferrite transformers are difficult to assemble to a circuit board requiring a lengthy manual assembly process by a skilled operator. This adds undesirable cost to the product and is difficult for large scale manufacturing.
The variation of the ferrite balun transformer effects the LO balun the most where the high impedance of a transistor gate creates a mis-match with the LO port. Typically, it is desired that the LO port have an impedance of 50 ohms. This impedance mis-match limits the bandwidth that the mixer can handle. Mixers with a poor impedance match at the LO port can only handle narrow bandwidths. In order to cover a wide bandwidth, several mixers could be used, one for differing bandwidth ranges. However, this would be very expensive and require excessive circuit board space. Several prior art mixer circuits are well known and have been described in the prior art.
While various mixers have been used, they have suffered from not being able to handle wide bandwidths with high values of IP3 (low inter-modulation products), being difficult to assemble, expensive to produce and requiring excessive circuit board space.
A current unmet need exists for a mixer that is compact, can be assembled at low cost and that can handle wide frequency bandwidths with low levels of third order inter-modulation products.